1. Field of the Invention
The present invention relates to an encoding apparatus and method for converting m-bit data words into n-bit code words. In addition, the present invention relates to a recording apparatus and method for recording a recording code string obtained by such encoding. Furthermore, the present invention relates to a decoding apparatus and method for decoding the recording code string.
2. Description of the Related Art
As an optical recording medium on or from which a signal is recorded or played back by radiating light, for example, so-called optical discs such as a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray Disc (BD) (registered trademark) are widely used.
For these currently widely used optical discs, mark edge recording, in which recording codes are defined by non-return-to-zero inverting (NRZI) and converted into non-return-to-zero (NRZ) codes before the recording, is performed.
In addition, in optical discs, because a tracking error signal is obtained from grooves and pits, it is necessary for a recording code to have a small number of low-range components. That is, since a tracking servo band is located in a lower range than the signal band of a recording code, if the recording code has a large number of low-range components, the low-range components of the recording code may be superimposed on the tracking error signal, which deteriorates the tracking servo characteristics.
Therefore, in optical discs of the related art, the absolute value of a digital sum value (DSV) in an NRZ code string to be recorded is controlled such that the absolute value is small, thereby reducing the number of the low-range components of a recording code.
For example, in CDs, an eight-to-fourteen modulation (EFM) code is used and a coding method is adopted in which a minimum run length of d=2 is satisfied between a 14-bit code word and a next code word and a predetermined 3-bit connection bit that reduces the absolute value of a DSV in a code string is selected and inserted.
In addition, in DVDs, a modulation code called “EFM plus” is used and DSV control is performed by selecting a code word from a main table or from a substitute table for a certain data word, the selected code word being one with which the absolute value of a DSV in a code string is smaller. This modulation code is described in, for example, Kees A. Schouhamer Immink, “EFMPlus: THE CODING FORMAT OF THE MULTIMEDIA COMPACT DISC”, IEEE Transaction on Consumer Electronics, Vol. 41, Issue 3, August 1995 and International Publication No. 95/22802.
In addition, in BDs, a modulation code called “17 Parity preserve/Prohibit (17PP)” is used and direct current (DC) control bits are periodically defined in the recording data format of a BD. A DC control bit of “0” or “1” that reduces the absolute value of a DSV in a code string is selected, and encoding is executed.
As a next-generation optical disc that follows on from the CD, DVD, and BD, which are widely used now, the present assignee has already proposed a bulk-recording (or simply bulk) optical disc described in, for example, Japanese Unexamined Patent Application Publication No. 2008-135144 and Japanese Unexamined Patent Application Publication No. 2008-176902.
Bulk recording herein refers to a technology for realizing large recording capacity by, as illustrated in FIG. 33, for example, performing multilayer recording with laser light that is radiated onto an optical recording medium (a bulk recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 and whose focus position is sequentially changed.
With respect to such bulk recording, a recording technology called a “microholographic method” is disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144. In the microholographic method, a so-called holographic recording material is used as a recording material of the bulk layer 102. As a holographic recording material, for example, a photo-polymerizable photopolymer or the like is widely used.
Microholographic methods are roughly classified into positive microholographic methods and negative microholographic methods.
A positive microholographic method is a method in which two light beams (a light beam A and a light beam B) that face each other are condensed at the same position to form minute interference fringes (hologram), which are used as recording marks.
A negative microholographic method is, in contrast to a positive microholographic method, a method in which interference fringes that have been formed in advance are deleted by radiating laser light and the deleted portions are used as recording marks. In a negative microholographic method, it is necessary to perform a process for forming interference fringes in the bulk layer in advance as an initialization process.
In addition, as a bulk recording method different from microholographic methods, the present assignee has also proposed a recording method in which voids are formed as recording marks, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2008-176902.
The void recording method is a method in which voids are recorded in the bulk layer 102 that is composed of a recording material such as a photo-polymerizable photopolymer by radiating laser light at relatively high power onto the bulk layer 102. As described in Japanese Unexamined Patent Application Publication No. 2008-176902, the formed voids each have a refractive index different from the other portions of the bulk layer 102 and therefore the reflectivity of light is increased at boundaries between the voids and the other portions. Therefore, the voids serve as recording marks, thereby realizing information recording by formation of the void marks.
Since a hologram is not formed in the void recording method, recording can be performed by radiating only a single light beam. That is, it is not necessary to condense two light beams at the same position to form a recording mark as in the case of the positive microholographic method described above.
In addition, in contrast to a negative microholographic method, it is not necessary to perform an initialization process, which is preferable.
It is to be noted that although an example in which precure light is radiated before void recording is described in Japanese Unexamined Patent Application Publication No. 2008-176902, voids can be recorded without radiation of the precure light.
As described above, various recording methods have been proposed for a bulk-recording optical disc recording medium. The recording layer (bulk layer) of such a bulk optical disc recording medium does not have a multilayer structure in an explicit sense, that is, for example, there is no plurality of reflective films. In other words, in the bulk layer 102, a reflective film and a pre-groove for each recording layer, which are typically included in a common multilayer disc, are not provided.
Therefore, it is difficult to perform focus servo and tracking servo with the structure of the bulk recording medium 100 illustrated in FIG. 33 when marks have not yet been formed during recording.
For this reason, in practice, the bulk recording medium 100 is provided with a reference reflection plane (reference plane) having pre-grooves illustrated in FIG. 34.
More specifically, pre-grooves (position guiding elements) formed of, for example, pits or grooves are formed in a lower surface side of the cover layer 101 in a spiral or concentric pattern, and a selective reflection film 103 is formed on the lower surface side. On the lower side of the cover layer 101 on which the selective reflection film 103 has been formed, the bulk layer 102 is stacked through an intermediate layer 104, which is composed of an adhesive material such as a ultraviolet (UV) curable resin.
By forming the pre-grooves from pits or grooves as described above, for example, absolute position information (address information) such as radius position information and rotation angle information is recorded. In the following description, a plane (a plane on which the selective reflection film 103 is formed in this case) in which such pre-grooves are formed and on which the absolute position information is recorded is called a “reference plane Ref”.
With the structure of the medium described above being adopted, not only laser light (hereinafter also referred to as recording/playback laser light or simply recording/playback light) for recording (or playing back) marks, but also servo laser light (also simply referred to as servo light) as laser light for position control are radiated onto the bulk recording medium 100 through a common objective lens as illustrated in FIG. 34.
At this time, if the servo laser light reaches the bulk layer 102, mark recording performed inside the bulk layer 102 may be adversely affected. For this reason, in the bulk recording method, laser light having a different wavelength range from the recording/playback laser light has been used as the servo laser light in the past, and the selective reflection film 103 having such wavelength selectivity that the servo laser light is reflected and the recording/playback laser light passes therethrough has been provided as a reflective film formed on the reference plane Ref.
On the basis of the above description, the operation during mark recording performed on the bulk recording medium 100 will be described. First, when multilayer recording is performed on the bulk layer 102 on which no pre-groove or reflective film is formed, the layer positions in the depth direction of the bulk layer 102 at which marks are recorded are determined in advance. In FIG. 34, an example in which a total of five information recording layer positions L, namely a first information recording layer position L1 to a fifth information recording layer position L5, are set as the layer positions (also referred to as mark formation layer positions or information recording layer positions) in the bulk layer 102 at which marks are formed is illustrated. As illustrated in FIG. 34, the first information recording layer position L1 is an information recording layer position L set at the top and the other information recording layer positions L are set on the lower layer side in order from L2 to L5.
When marks have not yet been formed during recording, focus servo and tracking servo for each layer position in the bulk layer 102 on the basis of reflected light of the recording/playback laser light are not performed. Therefore, focus servo control and tracking servo control of the objective lens during recording are performed by following the pre-grooves in the reference plane Ref with the spot position of the servo laser light on the basis of the reflected light of the servo laser light.
However, in order to perform mark recording, it is necessary for the recording/playback laser light to reach the bulk layer 102 formed on the lower layer side of the reference plane Ref and to be able to select the focus position in the bulk layer 102. Therefore, in this case, a recording/playback light focus mechanism (expander) for independently adjusting the focus position of the recording/playback laser light is provided in an optical system separately from the focus mechanism for the objective lens.
That is, by changing the collimation of the recording/playback laser light that is incident upon the objective lens with the expander that has been provided, the focus position of the recording/playback laser light is adjusted independently of that of the servo laser light.
The position of the recording/playback laser light in the tracking direction is automatically controlled by the above-described tracking servo of the objective lens using the servo laser light such that the position is directly below the pre-grooves in the reference plane Ref.
When the bulk recording medium 100 on which mark recording has already been performed is played back, it is not necessary to control the position of the objective lens on the basis of the reflected light of the servo laser light as in the case of recording. That is, during playback, the focus servo control and the tracking servo control of the objective lens are performed on mark strings formed at one of the information recording layer positions L (also referred to as information recording layers L or mark formation layers L in terms of playback) to be played back on the basis of the reflected light of the recording/playback laser light.